Surge chambers of the hydro-pneumatic type are normally used in conjunction with pumping plants and pipeline systems to avoid water hammer in the event of sudden pressure changes exerted on the fluid column in a pipeline. The surge chambers themselves are approximately half-full of fluid such as water and serve as stand-by pneumatic pumps which function when there is a power outtage or when the main pump system fails. Each surge chamber will deliver fluid to the pipeline system at a slowly decreasing rate, depending on the initial pneumatic air pressure in the chamber above the water surface. This feeding of fluid to the pipeline prevents the column of water in the pipeline system from "separating". In other words, a controlled gradually decreasing pressure is maintained on the water column in a manner analogous to maintaining pressure on a string of freight cars to prevent separation. Should separation occur, in either a water column or in the analogy of freight cars, when they reconnect, extensive damage can result. In the case of the fluid column, such reconnection of a separated water column is termed water "hammer".
The hydro-pneumatic surge chambers accordingly are very carefully designed with a known volume of pre-charged air and control outlet opening or orifice to carefully regulate the outflow of fluid from the chamber to a pipeline, so that column separation will not occur. Avoiding such column separation is accomplished by assuring that the pressure does not drop below some specified value.
Because the system using the surge chamber is essentially passive after a pump failure occurs, there will be a return surge or "oscillation" of the entire fluid column so that fluid will re-enter the surge chamber much like a geyser and recompress the air. If the outflow/inflow opening or orifice does not provide sufficient throttling, the air in the chamber may be compressed far above the initial operating pressure of the pumping system.
Further considerations in the design of surge chambers relate to assuring that no air is lost to the pipeline system from the surge chamber during outflow of fluid. If such air is lost, it would have to be rapidly replaced in time to enable the surge chamber to accommodate another power outtage or pump shutdown. A still further consideration in the design of such chambers, is to make sure that the fluid-air surface within the chamber or vessel is not agitated, since the fluid or water will tend to absorb the air under such agitation and thereby reduce the air volume which requires replacement.
In the past, surge chambers had their connections to a pipeline system in the side of the chamber or vessel with the reduced diameter orifice or nozzle portion located internally in a downcomer positioned within a few inches of the bottom of the tank, so that all of the surge volume can be employed. In fact, as early as 1917, I used a check valve as the communication connection between the interior of the surge tank in a pipeline with a hole in the flapper portion of the check valve. On the outflow or downsurge sequence, the check valve opened, allowing fluid in the surge chamber to run into the pipeline at a predetermined rate, depending upon the size of the connection line. On the return surge, the check valve closed and the hole in the flapper functioning as an orifice throttled the return surge. Later, I devised a swirl chamber as disclosed in U.S. Pat. No. 3,669,150 constructed within a surge chamber. At a later date, Fluid Kinetics Corp. used a venturi construction in the downcomer portion of the flow control fitting.
In most cases, surge tanks of potable water must have an internal coating for sanitary reasons. Any members or construction components within the surge tank must therefore also be coated, which becomes a vexing and expensive operation.